Dependency Inversion Using Abstract Service Tokens in Angular Providers

import { Injectable, Provider } from '@angular/core';
 
export abstract class StorageService {
  abstract getItem(key: string): string | null;
  abstract setItem(key: string, value: string): void;
}
 
@Injectable()
export class LocalStorageService extends StorageService {
  getItem(key: string): string | null {
    return localStorage.getItem(key);
  }
  setItem(key: string, value: string): void {
    localStorage.setItem(key, value);
  }
}
 
@Injectable()
export class MemoryStorageService extends StorageService {
  private store = new Map<string, string>();
  getItem(key: string): string | null {
    return this.store.get(key) ?? null;
  }
  setItem(key: string, value: string): void {
    this.store.set(key, value);
  }
}
 
export const provideStorage = (useMemory = false): Provider => ({
  provide: StorageService,
  useClass: useMemory ? MemoryStorageService : LocalStorageService
});